Photographic emulsions having high internal sensitivity



United States Patent No Drawing. Filed Aug. 27, 1965, Ser. No. 483,331 17 Claims. (Cl. 96108) This application is a continuation-in-part of Porter et al. application Ser. No. 110,109, filed May 15, 1961 (now US. Patent 3,206,313, issued Sept. 14, 1965).

This invention relates to photographic emulsions and elements and their preparation. In one of its aspects, this invention relates to photographic layers comprising photographic silver halide emulsions which have high internal sensitivity. In another of its aspects, this invention relates to photographic layers comprising photographic silver halide emulsions containing silver halide grains comprising a central core of chemically sensitized silver halide and, covering said central core, an outer shell of chemically sensitized silver halide.

Our copending application Ser. No. 110,109, filed May 15, 1961 (now US Patent 3,206,313, issued Sept. 14, 1965), discloses photographic silver halide emulsions having high internal sensitivity induced by chemical sensitization and the method of preparing such emulsions. The silver halide emulsions contain silver halide grains comprising a central core of chemically sensitized silver halide and, covering the central core, an outer shell of silver halide which can be chemically sensitized or substantially free of chemical sensitization. It has been found that the photographic silver halide emulsions in which both the central core and the outer shell of silver halide are chemically-sensitized exhibit increased photographic speed.

Accorrdingly, it is an object of this invention to provide photographic silver halide emulsions which exhibit good photographic speed.

Another object of this invention is to provide photographic silver halide emulsions containing silver halide grains which have high internal sensitivity induced by chemical sensitization.

Another object of this invention is to provide photographic silver halide emulsions containing silver halide grains comprising a central core of chemically sensitized silver halide and, covering the central core, an outer shell of silver halide which is treated with a sulfur sensitizer, a reduction sensitizer or a noble metal salt.

Still another object of this invention is to protect the high internal sensitivity of the photographic silver halide emulsion prepared according to this invention.

Still another object of this invention is to provide photographic elements comprising at least one photographic silver halide layer in which the silver halide grains have high internal sensitivity and comprise a central core of chemically sensitized siliver halide and a contiguous, outer shell of silver halide.

Other objects of this invention will become apparent from an examination of the specification and claims which follow.

According to this invention, it has been found that the above objects can be realized by blending a very fine grain silver halide emulsion with a chemically sensitized emulsion having considerably larger average grain distribution, holding this mixture for a sufficient time to produce a substantially homogeneous grain size distribution and chemically sensitizing the grains. On holding the composite emulsion for a suitable length of time (generally several hours) at a suitable temperature, there is obtained an emulsion which exhibits very low surface speed and fog, but very high internal speed. The amount of internal fog obtained can be controlled by the amount of surface fog obtained in the finish of the coarser-grained component of the blend. Upon sensitizing the composite emulsion after holding there is obtained a photographic silver halide emulsion having a chemically sensitized outer shell, as shown by significantly increased surface speed. In contrast, Example 2 shows that chemical sensitization without prior formation of the shell by holding does not give a chemically sensitized outer shell, as shown by the very poor photographic speed obtained after development in a conventional surface image type developer. This latter procedure is a conventional chemical sensitization of the type shown at column 2, lines 17-33 of Lowe et al. US. Patent 2,518,698, issued Aug. 15, 1950.

It is evident that the preparation of the internally sensitized emulsions of this invention involves two discrete components. The coarser-grained component is termed the core, for it appears that during the aforementioned period of holding, a ripening process occurs during which the finer-grained component partially dissolves and recrystallizes upon the surface of the coarser-grained silver halide. The original surface sensitivity and fog specks of the coarser-grained silver halide, or the core grains are thus buried, causing them to behave as internal specks. The finer-grained silver halide is termed the shell because it appears that these grains dissolve and recrystallize upon the surface of the core emulsion.

The core emulsions used in this invention comprise those which, when examined according to normal photographic testing techniques by coating a test portion of the emulsion on a transparent support, exposing to a light intensity scale for a fixed time between 0.01 and 1 second and development for 6 minutes at 68 F. in Developer A, as hereinafter defined, have a sensitivity greater than the sensitivity of an identical test portion of the same emulsion (measured at a density of 0.1 above fog), which has been exposed in the same way, bleached 5 minutes in an aqueous 0.3 percent potassium ferricyanide solution at 65 F., and developed for 5 minutes at 65 F., in Developer B, as hereinafter defined. Developer 'A is the usual type of surface image developer and Developer B is an internal developer having high silver halide solvent activity. The degree of internal sensitivity of the surface image emulsions is subject to variation. The surface image emulsion can have relatively little internal sensitivity, or it may have a fair amount-of internal sensitivity. Suitable surface image silver halide emulsions can be any of the conventional silver halide emulsions, although it has been found that silver bromide emulsions containing a relatively high content of bromide, e.g., at least mole percent bromide, are particularly useful.

These emulsions can contain up to about 10 mole percent of another silver halide, such as silver iodide. Silver bromide and silver bromoiodide emulsions are particularly useful in our invention. Such emulsions have been previously described by T rivelli and Smith in The Photographic Journal, vol. LXXX, July 1940 (pages 285- 288).

Before blending with the shell emulsions, the core emulsions are first chemically sensitized by any means suitable for this purpose, many methods having been previously described in the prior art. Chemical sensitization, as used herein, includes sensitization of the type described by Antoine Hautot and Henri Saubenier in Science et Industries Photographiques, vol. XXVIII, January 1957, pages 1-23 and January 1957, pages 57-65. Such chemical sensitization includes three major classes, viz., gold or noble metal sensitization, sulfur sensitization, such as by a labile sulfur compound, and reduction sensitization, i.e., treatment of the silver halide with a strong reducing agent which does not fog appreciably the silver halide but introduces small specks of metallic silver into the silver halide crystal or grain.

The core emulsions can be chemically sensitized by any of the accepted procedures. The core emulsions can be digested with naturally active gelatin, or sulfur compounds can be added, such as those described in Sheppard U.S. Patent 1,574,944, issued Mar. 2, 1926, Sheppard et al. U.S. Patent 1,623,499, issued Apr. 5, 1927, and Sheppard et al. U.S. Patent 2,410,689, issued Nov. 5, 1946.

The core emulsions can also be treated with salts of the noble metals, such as ruthenium, rhodium, palladium, iridium, and platinum. Representative compounds are ammonium chloropalladate, potassium chloroplatinate, and sodium chloropalladite, which are used for sensitizing in amounts below that which produces any substantial fog inhibition, as described in Smith and Trivelli U.S. Patent 2,448,060, issued Aug. 31, 1948, and as antifoggants in high amounts, as described in Trivelli and Smith U.S. Patents 2,566,245, issued Aug. 28, 1951 and 2,566,263, issued Aug. 28, 1951.

The core emulsions can also be chemically sensitized with gold salts as described in Waller et al. U.S. Patent 2,399,083, issued Apr. 23, 1946, and Damschroder et a1. U.S. Patent 2,642,361, issued June 16, 1953. Suitable compounds are potassium chloroaurite, potassium aurithiocyanate, potassium chloroaurate, auric trichloride and 2-aurosulfobenzothiazole methochloride.

The core emulsions can also be chemically sensitized with reducing agents, such as stannous salts (Carroll U.S. Patent 2,487,850, issued Nov. 15, 1949), polyamines, such as diethylene triamine (Lowe and Jones U.S. Patent 2,518,698, issued Aug. 15, 1950), polyamines, such as spermine (Lowe and Allen U.S. Patent 2,521,925, issued Sept. 12, 1950), or bis(,B-aminoethyl)sulfide and its water-soluble salts (Lowe and Jones U.S. Patent 2,521,926, issue-d Sept. 12, 1950).

The core emulsions can also be optically sensitized with cyanine and merocyanine dyes, such as those described in Brooker U.S. Patents 1,846,301, issued Feb. 23, 1932; 1,846,302, issued Feb. 23, 1932; and 1,942,854, issued J an. 9, 1934; White U.S. Patent 1,990,507, issued Feb. 12, 1935; Brooker and White U.S. Patents 2,112,140, issued Mar. 22, 1938; 2,165,338, issued July 11, 1939; 2,493,747, issued Jan. 10, 1950 and 2,739,964, issued Mar. 27, 1956; Brooker and Keyes U.S. Patent 2,493,748, issued Jan. 10, 1950; Sprague U.S. Patents 2,503,776, issued Apr. 11, 1950 and 2,519,001, issued Aug. 15, 1950; Heseltine and Brooker U.S. Patent 2,666,761, issued Jan. 19, 1954; Heseltine U.S. Patent 2,734,900, issued Feb. 14, 1956; Van Lare U.S. Patent 2,739,149, issued Mar. 20, 1956; and Kodak Limited British Patent 450,958, accepted July 15, 1936.

The core emulsions can also contain speed increasing compounds of the quaternary ammonium type of Carroll U.S. Patent 2,271,623, issued Feb. 3, 1942; Carroll and Allen U.S. Patent 2,288,226, issued June 30, 1942; and Carroll and Spence U.S. Patent 2,334,864, issued Nov. 23, 1943; or the polyethylene glycol type of Carroll and Beach U.S. Patent 2,708,162, issued May 10, 1955; or the quaternary ammonium salts and polyethylene glycols of Piper U.S. Patent 2,886,437, issued May 12, 1959; as well as the thiopolymers of Graham and Sagal, U.S. Patent 3,046,129, issued July 24, 1962 and Dann and Chechak U.S. Patent 3,046,134 issued July 24, 1962.

The shell emulsions useful in our invention can be prepared according to techniques which have been previously described in the prior art. Methods for preparing such emulsions have been previously described by Trivelli and Smith in The Photographic Journal," vol. LXXIX, May 1939, pages 330338. While any of the usual silver halides, such as silver chlorobromide, and the like, can be used in the shell emulsions, particularly useful results have been obtained with silver bromide emulsions, particularly silver bromide emulsions containing at least mole per-cent of silver bromide. Other halides, up to about 10 percent, can be used with the silver bromide emulsions. Silver bromide and silver bromoiodide emulsions (e.g., silver bromoiodide emulsions containing up to about 10 mole percent of iodide) have been found to be particularly useful in preparing the shell emulsions of our invention. Although the shell emulsions can, in some cases be chemically unsensitized, as shown in our copending application Ser. No. 110,109, filed May 15, 1961 (now U.S. Patent 3,206,313, issued Sept. 14, 1965), they are chemically sensitized in the practice of this invention. These shell emulsions can be chemically sensitized by any means suit-able for this purpose, including those techniques described herein for chemically sensitizing the core emulsions. For example, the shell emulsions can be chemically sensitized by treating with sulfur sensitizers, reduction sensitizers, noble metal salts and combinations thereof. Furthermore, the shell emulsions can be treated with additional addenda, for example, spectral sensitizers, speed increasing compounds, and the like, as described for the core emulsions.

In order to obtain the useful effects illustrated below, it is important that the grain size distribution of the shell emulsions be such that the average grain size is not greater than 0.4,u., and preferably about 0.1g. The average grain size distribution of the core emulsion should be such that the silver halide grain size has .a diameter greater than about 0.8 (e.g., 0.8 to about 1.5,u.).

In general, it has been found that particularly useful results can be obtained by blending about 0.25 to about 8.0 molar equivalents of the shell emulsion per molar equivalent of the core or surface-sensitive emulsion. When these blends are held for several hours, usually for about 24 hours, at room temperature the shell grains dissolve and deposit upon the surface of the core grains so that the resulting composite emulsion shows very high internal sensitivity, but very low surface sensitivity which is increased by chemically sensitizing, as described herein. Also, the resulting composite emulsion, which can be termed a covered grain emulsion, contains a population of silver halide grains which are more or less uniform in grain-size distribution, as contrasted with emulsion blends which contain at least two types of silver halide grains, which are separate and distinct in their physical and, frequently, photographic properties.

The following developing solutions are used in the characterization of the silver halide emulsions described in this application.

A N-methyl-p-aminophenol sulfate grams 2.5 Ascorbic acid do 10 Potassium metaborate do 35 Potassium bromide do 1 Water to 1 liter. pH, 9.6.

N-methyl-p-aminophenol sulfate grams 2.0 Sodium sulfite, desiccated do 90 Hydroquinone do 8.0 Sodium carbonate, monohydr-ate do 52.5 Potassium bromide do 5 Sodium thiosulfate do 10.

Water to 1 liter.

.in the following Table 1.

suits are as follows:

Ordinaryphotographic silver halide emulsions which have been chemically sensitized according to methods which have been previously described in the prior art tend to lose a good deal of their sensitivity upon storage, usually as a result of chemical interaction with the environment. On the other hand, the covered grain emulsions of this invention do not exhibit to such amarked degree the loss of their chemical sensitivity, since the internal chemical sensitivity centers are covered by a layer of protective silver halide. Moreover, these covered grain emulsion-s retain their high spectral sensitivity and do not exhibit the marked blue desensitization which is characteristic of many prior art spectrally-sensitized silver halide emulsions. Moreover, the covered grain emulsions of our invention have much improved reciprocity failure char- 'acteris-tics.

This invention can be further illustrated by the following examples of preferred embodiments thereof although it will be understood that these examples are includes merely for purposes of illustration and are not intended to limit the scope of the invention unless otherwise specifically indicated.

EXAMPLE 1 The photographic emulsions of this invention contain silver halide grains comprising a central core of chemically sensitized silver halide which central core is covered by a contiguous outer shell of silver halide which is also chemically sensitized. To illustrate, a medium grain gelatino-silver bromoiodide emulsion (9-812 mole ratio) having an average grain size of about 1.5 microns diameter is sulfur and reduction sensitized with 2 mg. of sodium thiosulfate and 8 mg. .of triethylene tetramine per mole of silver halide for 25 minutes at 55 C. to form the sensitized core emulsion. To one mole of the sensitized core emulsion is added 3 moles of chemically unsensitized fine. grain gelatino-silver bromoiodide emulsion containing approximately 1.2 mole percent iodide, and 24 g. of potassium bromide per liter of total melt. The mixture is held for 2 hours at 40 C. to form a shell of unsensitized silver halide on the sensitized core emulsion, i.e., a covered grain emulsion. The covered grain emulsion is 'then treated With 10 mg. of sodium thiosulfate and 40 mg. of triethylene tetramine per mole of silver halide and heated for 25 minutes at 70 C. to form a covered grain emulsion having a chemically sensitized shell of silver halide.

Samples of the sensitized core emulsion, the covered grain emulsion having a shell of unsensitized silver halide and the covered grain emulsion having a shell of sensitized silver halide are coated on separate conventional cellulose acetate film support at silver coverages of 800 mg./ ft. and gelatin coverages of 700 m-g./ft. Samples of each coating are exposed on an intensity scale sensitometer and recessed for 5 minutes in Kodak DK-SO De- .veloper, a surface image developer. The samples are fixed, washed .and dried in the usual manner and the photographic speed determined. The results are set forth Photographic speed, as indicated, is a function of the exposure necessary to give a .density of 0.1 above background fog, the speed of the sensitized core emulsion being taken as 100. The re- Table 1 Sample: Speed Sensitized core emulsion '0 Covered grain emulsion-unsensitized shell 1 Covered grain emulsion-sensitized shell 166 1 Virtually no image.

'sulfur compound and a gold salt or with a gold salt alone.

of supports in preparing photographic elements.

6 EXAMPLE 2 As already indicated, the chemical sensitizing agents must be added to the covered grain emulsions of this invention after the formation of the shell in order to obtain increased surface speed, i.e., in order to obtain a chemically sensitized silver halide outer shell. To illustrate, one mole of the sensitized core emulsion described in Example 1 is blended with 3 moles of the chemically unsensitized fine grain gelatino-silver bromoiodide emulsion also described in Example 1, 24 g. of potassium bromide per liter of melt, 10 mg. of sodium thiosulfate per mole of silver halide and 40 mg. of triethylene tertamine per mole of silver halide. The blend is held for 2 hours at 40 C. The blend is then coated on cellulose acetate film support, exposed, developed, fixed, washed and dried as described in Example 1. The photographic speed, determined as in Example 1, is as follows:

Table 2 Sample: Speed Sensitized core emulsion blended with fine grain emulsion and sensitizer simultaneously 1 By comparison of the results reported in the above table with those reported in Table 1 it can be seen that the addition of the chemical sensitizing agents during formation of the silver halide shell does not give a silver halide grain having a chemically sensitized outer shell.

If desired, the covered grain or CG emulsions of this invention can be further chemically sensitized or treated with sensitizing dyes or antifoggants according to methods Well known to those skilled in the photographic art. For example, the CG emulsions of our invention can be chemically sensitized, spectrally sensitized, stabilized, treated with gelatin plasticizers, hardeners and the like, according to the methods described in Luckey and Hoppe U.S. Patent 2,996,382, issued Aug. 15, 1961.

In the preparation of the silver halide dispersions employed for preparing the silver halide emulsions of this invention, there can be employed as the dispersing agent for the silver halide in its preparation, gelatin or some other colloidal material, such as collodial albumin, a polyacrylamide having a combined acrylamide content of 3 060% and a specific viscosity of 0.251.5 or an imidized polyacrylamide of like acrylimide content and viscosity as described in Lowe, Minsk and Kenyon U.S. Paten't 2,541,- 474, issued Feb. 13, 1951; zein as described in Lowe U.S.,Patent 2,563,791, issued Aug. 7, 1951; or a polymeric material which results from polymerizing a protein or a saturated acylated protein with a monomer having a vinyl group as described in Illingsworth, Dann and Gates U.S. Patent 2,852,383, issued Sept. 16, 1958. If desired, compatible mixtures of two or more of these colloids can be employed for dispersing the silver halide in its preparation. Combinations of antifoggants, sensitizers, hardeners, etc., can be used.

The photographic silver halides employed in the practice of this invention can be coated on a wide variety The supports employed are preferably flexible and can also be transparent. Typical supports are cellulose nitrate film, cellulose ester film, polyvinyl acetal film, polystyrene film, polyethylene terephthalate film, and related or resinous materials as well as glass, paper, metal and the like. Supports such as paper which are coated with a-olefin polymers, particularly polymers of a-olefins containing two or more carbon atoms, as exemplified by polyethylene, polypropylene, ethylene-butene copolymers and the like, can also be employed.

The photographic elements or CG emulsions described above can be used in difiusion transfer processes which utilize the undeveloped silver halide in the non-image areas of the negative to form a positive by dissolving the undeveloped silver halide and precipitating it on a receiving layer in close proximity to the silver halide emulsion layer. Such processes are described in Rott U.S. Patent 2,352,014, issued June 1944, and Land U.S. Patents 2,584,029, issued Jan. 29, 1952; 2,698,236, issued Dec. 28, 1954 and 2,543,181, issued Feb. 27, 1951; and Yackel et al. U.S. Patent 3,020,155, issued Feb. 6, 1962. They can also be used in color transfer processes which utilize the diffusion transfer of an image-wise distribution of developer, coupler or dye, from a lightsensitive layer to a second layer, while the two layers are in close proximity to one another. Color processes of this type are described in Land U.S. Patents 2,559,643, issued July 10, 1951, and 2,698,798, issued Jan. 4, 1955; Land and Rogers Belgian Patents 554,933 and 554,934, granted Aug. 12, 1957, International Polaroid Belgian Patents 554,212, granted July 16, 1957 and 554,935, granted Aug. 12, 1957; Yutzy U.S. Patent 2,756,142, issued July 24, 1956 and Whitmore and Mader U.S. patent application Ser. No. 734,141, filed May 9, 1958 (now abandoned).

The addenda described above for use in conjunction with the photographic emulsions described herein can be employed in emulsions designed for X-ray photography, or in non-optically sensitized emulsion, or orthochromatic, panchromatic, or infrared sensitized emulsions. They can be added before or after sensitizing dyes are added. These addenda can also be used in emulsions designed for color photography, for example, emulsions containing color-forming compounds or couplers, or emulsions to be developed in solutions containing such couplers, or emulsions of the mixed-packet type, such as described in Godowsky U.S. Patent 2,698,794, issued Jan. 4, 1955, etc.

The covered grain or CG emulsions described herein can be treated with sufiicient chemical sensitizer during the preparation to substantially completely fog the core emulsion component, and the resulting fogged internally sensitive grains used in the invention described in Luckey and Hoppe, U.S. Patent No. 2,996,382 mentioned above. The CG emulsions of our invention are also useful in producing reversal films, such as those described in Fallesen U.S. Patent 2,497,857, issued Feb. 21, 1950-.

The invention has been described in detail with particular. reference to preferred embodiments thereof, but it will be understood that variations and modifications can be effected within the spirit and scope of the invention as described hereinabove and as defined in the appended claims.

We claim:

1. A photographic silver halide emulsion containing silver halide grains having high internal sensitivity, said grains comprising a central core of chemically sensitized silver halide, and covering said central core, an outer shell of silver halide which is chemically sensitized subsequent to the formation of said outer shell.

2. A photographic silver halide emulsion as described in claim 1 wherein the central core of silver halide is chemically sensitized with a labile sulfur compound and a gold salt.

3. A photographic silver halide emulsion as described in claim 1 wherein the silver halide of the central core and the outer shell contain not more than 10 mole percent iodide and at least 90 mole percent bromide.

4. A photographic silver halide emulsion containing silver halide grains having high internal sensitivity, said grains comprising a central core of chemically sensitized silver halide and, covering said central core, an outer shell of silver halide which is treated with a sulfur sensitizer, a reduction sensitizer or a noble metal salt subsequent to the formation of said outer shell.

5. A photographic silver halide emulsion containing silver halide grains having high internal sensitivity, said grains comprising a central core of sulfur and reduction sensitized silver halide and, covering said central core, an outer shell of silver halide which is sulfur and reduc- 8 tion sensitized subsequent to the formation of said outer shell.

6. A photographic element comprising a support and at least one photographic silver halide emulsion layer containing silver halide grains having high internal sensitivity, said grains comprising a central core of chemically sensitized silver halide and, covering said central core, an outer shell of silver halide which is chemically sensitized subsequent to the formation of said outer shell.

7. A photographic element as defined in claim 6 wherein said central core of silver halide is chemically sensitized with a labile sulfur compound and a gold salt.

8. A photographic element comprising a support and at least one photographic silver halide emulsion layer containing silver halide grains having high internal sensitivity, said grains comprising a central core of chemically sensitized silver halide and, covering said central core, an outer shell of silver halide which is treated with a sulfur sensitizer, a reduction sensitizer or a noble metal salt subsequent to the formation of said outer shell.

9. A photographic element comprising a support and at least one photographic silver halide emulsion layer con taining silver halide grains having high internal sensitivity, said grains comprising a central core of sulfur and reduction sensitized silver halide and, covering said central core, an outer shell of silver halide which is sulfur and reduc tion sensitized subsequent to the formation of said outer shell.

10. A photographic element comprising a flexible support and at least one photographic silver halide emulsion layer containing silver bromide grains comprising a central core of chemically-sensitized silver bromide and a contiguous outer shell of silver bromide which is chemically sensitized subsequent to the formation of said outer Shell.

11. A photographic element comprising a support and at least one photographic silver halide emulsion layer containing silver halide grains containing not more than 10 mole percent of iodide and at least mole percent bromide, said grains comprising a central core of chemicallysensitized silver bromoiodide and a contiguous outer shell of silver bromoiodide which is chemically sensitized subsequent to the formation of said outer shell.

12. A photographic eelment comprising a support and at least one photographic silver halide emulsion layer containing internally, sulfur-sensitized silver halide grains containing at least 90 mole percent bromide, said grains comprising a central core of sulfur-sensitized silver bromide containing at least 90 mole percent silver bromide and a contiguous outer shell of silver bromide containing at least 90 mole percent bromide and which is chemically sensitized subsequent to the formation of said outer shell.

13. A photographic element according to claim 12 wherein said silver halide contains at least 90 mole percent silver bromide and not more than 10 mole percent silver iodide.

14. A photographic element comprising a support and at least one photographic silver halide emulsion layer containing internally, gold-sensitized silver halide grains containing at least 90 mole percent bromide, said grains comprising a central core of gold-sensitized silver bromide containing at least 90 mole percent silver bromide and acontiguous outer shell of silver bromide containing at least 90 mole percent bromide and which is chemically sensitized subsequent to the formation of said outer shell.

15. A photographic element according to claim 14 wherein said silver halide contains at least 90 mole percent silver bromide and not more than 10 mole percent silver iodide.

16. A photographic element comprising a flexible support and at least one photographic silver halide emulsion layer containing silver bromide grains comprising a central core of sulfur and gold-sensitized silver bromide and a contiguous outer shell of silver bromide which is chemically sensitized subsequent to the formation of said outer shell.

17. A process of preparing photographic silver halide emulsions comprising a central core of chemically-sensitized grains and a shell over said core of chemically-sensitized silver halide grains comprising digesting a mixture of chemically-sensitized silver halide grains having an average diameter of at least about 0.8,u together with unsensitized silver halide grains having an average diameter less than 0.4 1. until said mixture has a single population of silver halide grains of substantially average uniform diameter sized grains and chemically sensitizing said mixture.

References Cited by the Examiner UNITED STATES PATENTS OTHER REFERENCES Zhurnal Nanchnoi i Prikladnoi Fotografii i Kinematografii 5: 81-83, No. 2, 1960.

NORMAN G. TORCHIN, Primary Examiner. J. RAUBITSCHEK, Assistant Examiner. 

1. A PHOTOGRAPHIC SILVER HALIDE EMULSION CONTAINING SILVER HALIDE GRAINS HAVING HIGH INTERNAL SENSITIVITY, SAID GRAINS COMPRISING A CENTRAL CORE OF CHEMICALLY SENSITIZED SILVER HALIDE, AND COVERING SAID CENTRAL CORE, AN OUTER SHELL OF SILVER HALIDE WHICH IS CHEMICALLY SNESITIZED SUBSEQUENT TO THE FORMATION OF SAID OUTER SHELL. 